The present invention relates to heat-resistant and oxidation resistant sintered metal bodies suitably used as catalyst carriers, heaters, catalytic converters, etc. The invention relates also to a method for manufacturing the bodies.
In the manufacturing process, the bodies are susceptible to oxidation. The body is typically subjected to procedures in which the catalyst components are incorporated onto or into it. In these procedures, the body can be in contact with corrosive materials, for example, when noble metals are incorporated, they are usually in acid solutions of their salts. Furthermore, in actual use of the catalyst system, the bodies are exposed to corrosive and high temperature environments. Additionally, in these applications, the bodies are subjected to frequent and rapid heat-cool cycles. As a result, the bodies are prone to oxidation which shortens their life and effectiveness.
In recent years honeycomb structures produced by winding a metal foil in a corrugated form have come to be used as a catalyst or a catalyst carrier for purification of nitrogen oxides, carbon monoxide, and hydrocarbons present in the exhaust gases emitted from internal combustion engines of automobiles, etc. As such honeycomb structures, there are known those described in Japanese Patent Publication No. 23138/1983.
However, in the foil type metal honeycomb structure described in Japanese Patent Publication No. 23138/1983, the catalyst layer cannot be tightly adhered to the metal substrate with thin oxide layer formed thereon because of its low porosity, and the catalyst layer (which is a ceramic) readily peels off the metal substrate due to the difference in thermal expansion between the ceramic catalyst layer and the metal substrate. Further, a telescoping phenomenon readily occurs during the driving cycle in which the metal-to-metal joint breaks and the metal substrate deforms in such a manner that it protrudes in the direction of the flow of gas. This may disturb safe running of the vehicle. Furthermore, in the manufacture of the foil type metal honeycomb, the yield of the foil rolling step is low, resulting in a high production cost.
Honeycomb structures manufactured by forming metal powders and sintering the formed body are also disclosed. Such honeycomb structures have been proposed in, for example, Japanese Patent Laid-Open No. 310942/1988, Japanese Patent Publication No. 6974/1982, and Japanese Patent Laid-Open Nos. 57803/1982 and 57804/1982.
Japanese Patent Laid-Open No. 310942/1988 (also U.S. Pat. No. 4,758,272) discloses a honeycomb structure having a composition comprising, as analyzed in weight percent 5-50% of Al, 30-90% of Fe, 0-10% of Sn, 0-10% of Cu, 0-10% of Cr, and not more than 1% of Mg and/or Ca, a porosity of about 25-75%, and a specified cell density.
Japanese Patent Publication No. 6974/1982 discloses a honeycomb structure manufactured by binding a large number of small honeycomb components made of powders of catalytically active substances (nickel, copper, chromium) which act as catalysts for exhaust gas purification. In the honeycomb structure, however, the oxidation treatment is conducted at a low temperature to obtain a catalytic activity, and accordingly the honeycomb structure has insufficient heat resistance. Further this honeycomb structure is not a monolithic body and is broken due to the vibrations applied during the practical use.
Japanese Patent Laid-Open Nos. 57803/1982 and 57804/1982 each disclose a metal honeycomb structure manufactured by mixing metal powders, a thermosetting binder, colloidal silica, etc., forming the mixture into a honeycomb shape by extrusion, and hardening and sintering the shaped body. The documents, however, mention, as metal powders, only a SUS powder, Al, Cu, Ti and Zr and make no mention on a Fe-Cr-Al system and additive elements, and the obtained metal honeycomb structures are poor in oxidation resistance.
Meanwhile, Japanese Patent Laid-Open No. 159355/1989 discloses a heat-resistant cast steel of Fe-Cr-Al system. The technique disclosed in the document, however, relates to a cast steel and does not lie in a sintered metal body.
U.S. Pat. No. 4,992,233 relates to methods of producing porous, sintered bodies of FeAl with optional additions of Sn, Cu, and Cr, by a series of firing operations involving firing the green body in an oxidizing atmosphere to remove the binder and oxidize some of the metal, followed by firing in a reducing atmosphere to reduce the oxidized metal, followed by firing in a non-oxidizing atmosphere to sinter and densify the body.
The present invention is intended to provide a sintered metal body excellent in oxidation resistance at high temperatures, heat resistance, corrosion resistance, etc. and a method for manufacturing such a sintered metal body.